Nonlinear parameters of objects to be measured such as materials, specimens, samples and bodies may be measured using ultrasonic waves, having single frequencies, which are a kind of elastic wave. This measurement uses nonlinear behavior of elastic waves, and micro-structural transmutation of objects to be measured caused by degradation such as corrosion and fatigue may be evaluated using nonlinear properties of the elastic waves.
Specifically, a harmonic component is generated in addition to a basic frequency component by nonlinear properties of elastic waves while the elastic waves having single frequencies are propagated through an object to be measured.
A nonlinear parameter of an object to be measured may be measured using the nonlinear properties of elastic waves by measuring sizes of a basic frequency component and a secondary harmonic component of a signal propagated to the object and by calculating a relative ratio between the sizes of the basic frequency component and the secondary harmonic component. The relative ratio is typically defined as a nonlinear parameter β and is expressed by the following Equation 1.
                    β        =                              8                          xk              2                                ⁢                                    A              2                                      A              1              2                                                          [                  Equation          ⁢                                          ⁢          1                ]            
Here, A1 and A2 are amplitudes of the primary (basic) frequency component and the secondary harmonic component, respectively, k is a wave number, and x is a propagation distance. In addition, the primary frequency component means an ultrasonic signal having the same frequency as the basic frequency component, and the secondary harmonic component means a high-frequency ultrasonic signal having a frequency twice the basic frequency.
FIG. 1A is a conceptual diagram for explaining a method of measuring a nonlinear parameter of an object to be measured according to the related art. FIG. 1B is a graph illustrating an ultrasonic signal output from the object to be measured according to the related art.
Referring to FIGS. 1A and 1B, when an ultrasonic signal having a single frequency component is incident on an object to be measured 20 through a probe 10, an ultrasonic signal having a secondary harmonic component of the single frequency component is output as an ultrasonic wave to be detected by a probe 30, in addition to the ultrasonic signal having the single frequency component, due to degradation generated inside the object to be measured 20.
In the signal detected by the probe, a basic frequency component having an amplitude of A1 and a frequency of f0, and a secondary harmonic component having an amplitude of A2 and a frequency of 2f0 are detected. Accordingly, a nonlinear parameter β may be calculated using the above Equation 1, and a degradation degree of the object to be measured may be evaluated by measuring and comparing nonlinear parameters before and after the generation of degradation.
The nonlinear parameter β has to be measured with consideration for only a harmonic component generated by an object to be measured. However, since harmonic components generated by electrical systems including the probe as well as the harmonic component generated by the object to be measured may be considered when the nonlinear parameter is actually measured, it is difficult to accurately measure the nonlinear parameter.
For example, when the probe is a contact probe coming into contact with an object to be measured, an unnecessary wavelength may be generated due to an irregular contact pressure between the probe and the object to be measured and thus a nonlinear component may be generated. For this reason, it is difficult to accurately the nonlinear parameter of the object to be measured.
Therefore, nonlinearity of the object to be measured may not be accurately measured due to addition of the nonlinear component generated by the above problems, besides the nonlinear parameter of the object to be measured.